Two Domains - One Problem

I have two domain names. One is keyword-rich, and one is memorable. How should I apply them both to the same site for maximum SEO? Right now I can't decide between two nearly-identical sites (with different text), a 301 redirect from one to the other (and which one to which one), moving the site to the keyword-rich domain, or making the keyword-rich site talk about the other.

Suggestions?

Thanks,
Peter

EDIT: The site in question is brand new. No current SEO to worry about.

 

I will also watch this thread.... I have also 2 domains... one is short but its a .US and other other is a long but its a .COM.. am planning to redirect the .US to .COM...

both are same names except that i added an "online" to the .COM domain....

 

Any SEO experts out there?

 

301 is the way to go. Check out seomoz.org/blog/whiteboard-friday-which-way-did-he-go

 

Do you have backlinks/PR to/on either of them yet?

If yes, use the one with the backlinks or PR as primary.

301 is the correct way to go when you decide.